1. Field of the Invention
This invention relates to aqueous dispersions of finely divided solids and, more particularly, to aqueous dispersions of finely divided solids that are stabilized against the destructive effects of freeze/thaw cycles.
2. Discussion of the Prior Art
It is well recognized that aqueous dispersions of finely divided solids are somewhat fragile in the sense that care must be taken to maintain a stable dispersion in which the particles, depending upon their density, neither settle to the bottom nor float to the top of the aqueous medium. Most generally, a surfactant is used to aid in establishing a stable aqueous dispersion and it is selected to have affinity for the dispersed particles at one end of its structure and to be hydrophilic at the other end of its structure. This provides for a basic compatibility between the dispersed particles and the water which, in a sense, can loosely be visualized as "hydrating" the dispersed particles.
A particularly troublesome problem arises when an aqueous dispersion is shipped or stored during the winter months and the dispersion is exposed to subfreezing temperatures. Generally it may be expected that when an otherwise stable dispersion is exposed to one or more freeze/thaw cycles, the dispersion will be "broken". This means that after the frozen dispersion is thawed, the particles will separate, often irreversibly, from the aqueous medium. As will be discussed in more detail below, it is believed that the reason the dispersion becomes unstable upon freezing is that the freezing process withdraws the "water of hydration" from the dispersed particles causing them to lose their mutual repulsion and coalesce.
The state of the art methods used to prevent the destruction of a stable dispersion from freeze/thaw cycles are quite simple. First, it is obvious that the dispersions may be shipped and stored in heated environments to prevent the ambient temperature from falling below the freezing temperature of the aqueous dispersion. Second, "anti-freeze" agents, such as alcohols and glycols, may be added to the aqueous dispersion in sufficient amounts to depress the freezing point to a temperature below that at which the aqueous dispersion will be exposed. Both of these methods have their drawbacks. For example, maintaining a suitable ambient temperature during shipping and storing requires special handling; it is somewhat expensive; and it is subject to human error, as when a container of an aqueous dispersion is accidentally left standing on a loading dock in cold weather.
Anti-freeze agents that depress the freezing point of the aqueous dispersion have several disadvantages in that they add an expense; some of them may impart toxicity to the aqueous dispersion; and, of particular concern depending upon the nature of the aqueous dispersion with which they are used, the anti-freeze agents when added in sufficient quantities to significantly depress the freezing point, may adversely effect the stability of the aqueous dispersion. In the latter instance, the useful properties of the dispersion and its commercial utility may be lost.
Occasional references to other methods for stabilizing an aqueous dispersion from the effects of freeze/thaw cycles can be found in the literature. For example, U.S. Pat. No. 4,053,443 discloses that a pipe joint sealing compound comprised of an aqueous dispersion of fluorocarbon particles can be freeze/thaw stabilized by the addition of a combination of sodium hydroxide and monoethanolamine. U.S. Pat. No. 3,879,302 claims the use of freeze/thaw stabilizing compounds selected from the group comprising silanes and silizanes (sic) to stabilize aqueous dispersions of polytetrafluoroethylene sealing compositions.